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Discerning Desmids

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This summer, in partnership with the Initiative for Sustainability and Energy at Northwestern (ISEN), Science in Society will profile four innovators in the areas of energy and sustainability—researchers who are harnessing the power of science and engineering to better understand and solve some of the many challenges facing our planet. This week we feature assistant professor of materials science and engineering and ISEN-award recipient Derk Joester.

The best engineers aren’t always found in the lab. Instead, scientists often find their inspiration in nature—organisms that, over billions of years, have evolved and developed elegant and sustainable ways of solving problems.

Joester and his team are looking to nature for help in solving a very modern problem—safely removing dangerous elements from nuclear waste. Their inspirational engineer? A type of algae known as desmids, which have the unique ability to isolate and crystallize the element strontium. Replicating the algae’s process or even using the algae itself to remove radioactive strontium from nuclear waste could provide a sustainable solution to storing this waste, or even cleaning up the environment after a nuclear accident.

We spoke with Joester and Minna Krejci, PhD student and leader of the lab’s desmid work, to learn more.

A type of algae known as desmids may provide a unique way of removing harmful elements from nuclear waste (Image provided by the Joester Laboratory of Northwestern University)Your lab looks to living organisms for “creative, efficient solutions to engineering problems.” What you mean by this?Derk Joester (DJ): We look at organisms in great detail to understand better how they solve problems they encounter. Organisms have had a long time, over the course of evolution, to find solutions. These solutions are frequently very energy efficient [and] make the best out of a limited set of tools, all under conditions that are essentially sustainable, because they need to [use] the energy they have. In the case of the algae we’re looking at, they work with sunlight. Everything they do doesn’t depend on the consumption of a lot of fuel.

It is this efficiency and sometimes very, very clever solutions to problems that we’re after. The way we think about it is that we reverse-engineer the process. We try to understand the biological process as much as we can, so that [we can], in the long-term, try to translate them into something we can use. So it’s catching up with a really, really advanced engineer, essentially.

One of the problems your lab works on is bioremediation. How does this work?DJ: When there is something in the environment that is dangerous to us—a radioactive compound, or a heavy metal, for example, from mining—we [very frequently] only have a brute force approach [to removing it]. We take up all the soil or collect all the water and, [using] a lot of energy and other resources, we try to separate that contaminant so that it doesn't get into the aquifers, the groundwater, our drinking water, our food, or into the air that we breathe.

There is a branch of science where people look at more energy-efficient, sustainable ways of doing the same thing. Bioremediation encompasses the use of either microorganisms—bacteria or algae—[or] plants to extract these contaminants from the ground. Many times, because plants have roots and they take up a lot of things, they…can’t simply excrete what they don’t like; they have to store it in a harmless form. If they just excreted it, they would take it right back up. [Consequently], many plants or algae have a mechanism to store things that are dangerous to them. We call that sequestration.

We can use this. People have, for example, taken tobacco plants, planted them on contaminated soil, and the plants will take up everything and store certain things in their leaves. You harvest the leaves, burn them or dry them, and then you have that contaminant in a very concentrated form—something that would require a lot more energy and heavy equipment if we were to take it from the soil directly.

We follow a similar approach with the desmids, the algae that we work with. They take up barium and strontium just as part of their normal metabolism, [and] then they put [them] into crystals. The idea [behind our research] was simply to see whether we could make the desmid take up more strontium, and to understand better why it is able to take up strontium and barium and put it away in that particular form, because that is unusual.

Derk JoesterWhy is finding a bioremediation method for strontium so important? DJ: Strontium is an element and, like many elements, occurs as a number of isotopes. (Isotopes are forms of the same element that contain different numbers of neutrons but the same number of protons.) A lot of naturally occurring strontium is harmless. In Europe, they even use it for osteoporosis therapy.

Strontium-90 (an isotope of strontium), however, is what we call a radioisotope. It has a couple of neutrons more than the stable isotopes, and that makes it fall apart. On average, every thirty years, half of the strontium will break apart. When it breaks apart, it will shoot out an electron, and that electron has very high energy. If it hits something along the way, then it can cause damage. We call that ionizing radiation.

Strontium-90, which is formed during fission in nuclear reactors, is dangerous to us because of the radioactive radiation. It is particularly dangerous because, in our body, strontium is treated [like] calcium and [it] goes to the bones. Strontium is very stable in the bones, so it stays there. So, for the entire rest of your life, you’re exposed to that radioactive radiation, and that is what really causes the trouble—a cancer risk, essentially.

In any kind of nuclear reactor, there is a small amount of strontium-90 that is formed as part of the regular fission process. In the fuel rods—the ones you heard a lot about when they started melting in Fukushima—there is a very small amount strontium-90, less than one percent. Ultimately, these fuel rods are spent and then they are reprocessed or just stored, and the strontium-90 is in there. [It] becomes part of the nuclear waste and because of its particular radioactive properties we consider it a high-level radioactive waste—something we really want to store safely away from where humans could be exposed to it.

But the total amount of strontium is really very small, so it’s a matter of sorting your waste into the small amount of things that take a lot money to store and separating them from the rest, because you really multiply the cost by the volume. If you can make the volume smaller, the price gets dramatically smaller.

The real problem is that many times it is difficult to pick out the strontium, because there is much more calcium present. [The algae we’re studying] have figured out a way to deal with this situation, and that’s what we’re interested in. What kind of mechanism do they use that allows them to pick just the strontium and the barium, put them in solid form that can be easily collected, and not bother with the calcium? Now we think we understand this mechanism a lot better.

How does it work? Minna Krejci (MK): [The parts of the algae’s cell] we’re interested in are called vacuoles—fluid-filled sacs, essentially. When barium, calcium and strontium are taken up into the cell and moved into the vacuole, there doesn’t seem to be a step along that way where the cell distinguishes between them. It’s not that it allows barium [and strontium] into the vacuole but not calcium.

The key step seems to be the mineralization process. [There is a] high concentration of sulfate [already in the vacuole] that leads to the precipitation of the compounds that have the lowest solubility, meaning that they don’t want to be in solution. (In chemistry, precipitation is the process by which a compound comes out of solution—a liquid mixture—and becomes a solid.)

[In this case,] the barium sulfate and strontium sulfate don’t want to be in solution, in comparison to the calcium sulfate. We’re trying to figure out at what level the cell actually has control over that process.

We can modify the medium [in which] we grow the algae to let them take up and mineralize more [strontium sulfate], basically by changing the concentrations of [the barium and strontium] in the medium. Raising the strontium to barium ratio led them to take up more strontium and mineralize more strontium sulfate in the crystals. Overall, they were sequestering more, and that’s what we would be interested in eventually if we use them to clean up radioactive waste.

Minna KrejciSo the more strontium you added to their growing environment, the more they took up? MK: Pretty much. The slight complication is that barium is necessary to lower solubility. Barium sulfate has very, very low solubility—it doesn’t want to be in solution. The strontium sulfate’s [solubility] is slightly higher—it doesn’t mind being in solution, but it can also precipitate out. Calcium sulfate is even higher—it has no problem being in solution. When the barium sulfate precipitates, it actually pulls with it some of the strontium. You [need] some amount of barium so that the solubility is low enough that the mineral forms at all, but we [also need] enough strontium so that we’re mineralizing a significant amount. By balancing solubility with the right amount of strontium…we were able to optimize.

But this was a proof of concept in that it doesn’t necessarily help us that much with actual bioremediation. We [wouldn’t] actually modify the amount of strontium in [contaminated water]. This just tells us more about [the process], and gives us clues as to more elegant ways that we could engineer the system to take up more strontium eventually.

Do you envision using the algae directly to separate out nuclear waste and to clean up accidents? Or would they inspire a new technology?DJ: We know from very old work—not from our own work—that desmids seem to have a high tolerance to radiation. That at least creates the possibility that you could use the algae directly to isolate [strontium]. But there are lots of questions that are not answered yet. How tolerant are they? Will they take [strontium] up quantitatively so that you could safely release the [contaminated] water? How tolerant are they to the other things that are in nuclear waste? These are questions that we have to answer before we can make even educated guesses as to whether they can be used.

My intuition is yes, this could be a way of going about it, but it will certainly take more research to be sure, and there is also ample room for improvement in the process. We haven’t looked at large-scale purification. Everything we do is lab scale. So there are lots of steps along the way before we can really make promises.

The beauty of algae in principle is that all they need is sunlight and water. And they grow relatively fast. So you could, in principle, take them anywhere in the world and, as long as there is light, they will grow and multiply. You [wouldn’t] have to move a lot of heavy equipment. I think it will be a very elegant system if it really works.

Your work has been partially funded by the Initiative for Sustainability and Energy at Northwestern (ISEN). How has this funding furthered your research? DJ: The booster grant from ISEN was specifically for [Minna’s work], so she was partially financed from that grant.

Another part was that we had an undergraduate who started building a bioreactor with the idea that we could test, on a slightly larger scale, how efficient the [strontium] uptake would be, and also test [desmids] in the presence of radioactivity. That reactor has been developed, but we haven’t done the radioactive experiments yet.

The ISEN grant will be very critical, once we’ve done the radioactive experiment, to attract more funding, especially from places like the National Science Foundation. The engineering division would be the place to fund the real translational part—making this into a real application.

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